The present invention relates to an optically transparent body of high density, polycrystalline alumina wherein at least one of the major surfaces is polished with a particular type molten fluxing agent for increased optical transmission. A tubular form of the glaze polished material when used as the light transmissive envelope for an improved high intensity electric discharge lamp provides a higher light output than is conventionally obtained with the unpolished material. Such improved envelope material is particularly useful in high intensity sodium vapor lamp constructions utilizing a grade of the polycrystalline alumina which already exhibits superior optical transmission in the unpolished condition as compared with earlier polycrystalline alumina ceramic.
The original polycrystalline alumina material made according to U.S. Pat. No. 3,026,210 (Coble), assigned to the assignee of the present invention, is still widely used for the light transmissive envelope in high intensity electric discharge lamps. This polycrystalline alumina ceramic is characterized by a relatively uniform large grain size and can be prepared with a minimal of secondary phase magnesia-alumina spinel at the grain boundaries. In U.S. Pat. No. 4,150,317, also assigned to the present assignee, there is disclosed a further improved optical grade of said alumina ceramic consisting essentially of relatively uniform equiaxed grains of alumina containing no more than about 150 ppm magnesium with essentially no porosity and secondary phase. Said material exhibits increased in-line transmission and is further characterized by the substantial absence of grain-growth promoting impurities and a pore volume fraction no greater than about 10.sup.-2. The average grain size of said material is approximately 26 microns diameter with substantially all grains having an average diameter in the range extending from about 20 microns diameter up to about 35 microns diameter. Whereas the original Coble material contained a small but effective amount up to about 0.5 weight percent magnesia content, a reduction in the magnesia content was found to impart greater control over grain growth as well as pore removal for less light scattering in the sintered product. A still more recently issued U.S. Pat. No. 4,285,732, also assigned to the assignee of the present invention, discloses a further improved grade of said optically transparent ceramic consisting essentially of alumina, magnesium and a component select from the group consisting of zirconium, hafnium and mixtures thereof. Magnesium is present in said further improved alumina material at amounts equivalent to from about 0.03% MgO by weight to less than about 0.15% MgO by weight of said sintered material, zirconium being present at amounts equivalent to from higher than about 0.002% ZrO.sub.2 by weight up to about 0.07% ZrO.sub.2 by weight of said sintered material, and hafnium being present at amounts equivalent to from higher than about 0.003% HfO.sub.2 up to about 0.12% HfO.sub.2 by weight of the sintered material. Mixtures of these additives are said to comprise all ratios of zirconium and hafnium in amounts equivalent to from higher than 0.002% by weight up to about 0.12%.
It is also known to improve optical transmission in the foregoing polycrystalline alumina materials by a flux polishing treatment as taught in U.S. Pat. Nos. 3,935,495; 4,033,743; and 4,079,167, also all assigned to the present assignee. The unpolished ceramic material is immersed in a molten inorganic flux selected from the group consisting of alkali metal salts and binary oxide systems having an alkali metal oxide constituent which dissolves the surface alumina grains and produces a relatively smooth surface appearance. In said treatment, the high spots of the individual surface alumina grains are reduced without materially etching the grain boundaries and the flux residue is removed from the treated ceramic generally by acid washing at near ambient temperatures. This selective etching process is carried out in the molten inorganic flux bath at temperatures up to around 1000.degree. C. with more elevated temperatures producing undesirable grain boundary etching accompanied by excessive volatilization of the fluxing agent being used. The preferred flux material is a eutectic composition of sodium borate salt at molar ratios between two moles B.sub.2 O.sub.3 up to four moles B.sub.2 O.sub.3 per one mole Na.sub. 2 O. Certain difficulties have been experienced with said prior art molten flux treatment, however, including a need to contain the molten flux in platinum vessels along with contamination of the molten flux by surface impurities contained in the alumina bodies being polished. The latter problem makes it difficult to control the treatment process while having to conduct said process in a container of the molten flux is not adaptable to using automated handling equipment for this task.
It is desirable, therefore, to carry out the flux polishing treatment in a manner avoiding the foregoing difficulties. By eliminating a molten flux bath for said treatment, it is further possible to reclaim polycrystalline alumina arc tubes which have been contaminated during the manufacture of high pressure sodium vapor lamps. It would also be desirable to polish polycrystalline alumina arc tubes by treating only the exterior major surface and thereby avoid having to remove flux residue from the tube interior. The polishing can also be used to alter the physical dimensions of alumina parts being fitted together in various product applications.